The IEEE 802.11e standard provides modifications to the baseline IEEE 802.11 standard for the support of an admission control function at the access point (AP) of a WLAN. The standard provides the mechanisms by which a station (STA) can request admission of a traffic stream (TS) and by which the AP can notify the STA of the admission or rejection of the request. In the initial 802.11e draft, admission control functionality existed mainly within the STA. In a later release, this functionality has been moved to the AP and is left unspecified. As the AP decides whether to accept or deny a request for admission, the algorithm used for this determination is not specified in the standard and is left for proprietary implementation.
With respect to the AP radio resource management (RRM) admission control function, if the AP supports admission control, it uses admission control mandatory (ACM) flags advertised in the enhanced distributed coordination function (EDCF) parameter set element to indicate whether admission control is enabled for each of the access categories (ACs). In the 802.11e standard, admission control is mandatory for Voice (AC_VO) and Video (AC_VI) access categories. However, the Background (AC_BK) and Best Effort (AC_BE) access categories do not require admission control.
While the minimum contention window (CWmin[AC]), maximum contention window (CWmax[AC]), arbitration interframe space (AIFS[AC]), and transmit opportunity (TXOP[AC]) limit parameters may be adjusted over time by the AP, the ACM bit is static for the duration of the lifetime of the basic service set (BSS). Upon reception of an add traffic stream (ADDTS) message from a STA with QoS functionality (QSTA), the AP (also referred to as a QAP) can decide to accept or reject the request and inform the QSTA of the decision.
With respect to QSTA admission control, each channel access function maintains two variables: Admitted_Time and Used_Time. The Admitted_Time and Used_Time variables are set at association time to zero. Admitted_Time is defined as the maximum amount of medium time that the QSTA can use during a one second period. The Used_Time is maintained by the QSTA and indicates the amount of medium time that the QSTA has used. At each successful or unsuccessful transmission attempt by the QSTA, Used_Time is updated by the total time required to transmit the frame, including the received acknowledge frame and the overhead in accessing the channel (SIFS interval). At one second intervals, the Used_Time is reset to 0 if it is less than or equal to the admitted time.
The STA may subsequently decide to explicitly request admission for a specific AC, which is associated with a specific IEEE 802.1D priority. In order to make such a request, the STA transmits a traffic specification (TSPEC) element contained in an ADDTS request management frame with the following fields specified (i.e., non-zero): nominal MAC service data unit (MSDU) size for the current traffic stream in bytes; mean data rate, the average data rate in bits per second (bps); minimum physical (PHY) rate, the desired minimum PHY rate in bps; and surplus bandwidth allowance, the excess allocation of time and bandwidth over and above the stated application rate requirements. It is noted that these fields are all obtained from operations and maintenance (OA&M) data. The Medium_Time field is not used in the request frame and is set to zero. The QSTA then waits for an ADDTS response message from the QAP.
Once the ADDTS response message is received, the QSTA evaluates it to determine if the admission request was approved. If the QAP allows the admission request, then the Medium_Time value (the transmission time available for a traffic stream) is obtained from a TSPEC element contained in the ADDTS response frame and the Admitted_Time variable is set equal to the Medium_Time value.
The QSTA then is free to commence frame transmission for the admitted traffic stream. The Used_Time is updated each time a frame transmission is initiated as follows:Used_Time=Used_Time+FrameExchangeTime  Equation (1)where FrameExchangeTime equals the time required to transmit the frame plus one acknowledgement (ACK) frame plus one short interframe space (SIFS) interval. Every second, the Used_Time is compared with the Admitted_Time:Used_Time=max((Used_Time−Admitted_Time), 0)  Equation (2)
When the Used_Time reaches or exceeds the Admitted_Time value, the corresponding channel access function no longer transmits using the EDCF parameters for that AC, until a future interval where the Used_Time reaches zero again. The 802.11e standard allows the STA to temporarily replace the EDCF parameters for that channel access function with those specified for the access categories Best Effort (AC_BE) or Background (AC_BK) in order to continue transmissions at a lower priority, provided no admission control is required for those access categories.
The STA or AP may choose to tear down an explicit admission request at any time. In order for the STA to tear down an explicit admission, the STA transmits a delete traffic stream (DELTS) frame containing the TSPEC element to the AP. If the STA receives a management notification frame with the action code set to DELTS, it disables the use of the specified AC. If an explicit admission is torn down, the STA disables the use of the AC. The Admitted_Time and the Used_Time values for the AC are then both set to zero.
According to the standard, some ACs can be admitted without an admission control procedure. A STA transmits an ADDTS request to the AP to request admission of traffic in any direction (uplink, downlink, or bi-directional) for an AC that requires admission control. The STA may also have the option to transmit un-admitted traffic under the ACs for which the AP does not require admission control. Therefore, if a STA desires to send data without admission control using an AC that mandates admission control, the STA can use an AC that corresponds to a lower priority and does not require admission control.